FullBlogPlugin: Permission fixes for when using fine-grained permission (such as the Draft sample plugin).

The archive listing did not check for fine-grained permissions at all. Now it does.

The sidebar content (periods, categories, categories - with totals) are now correctly filtered for permissions. Moved the method from 'model' to 'core' as it did not involve the database directly, was not really needed by any other model code, and any code involving perm and users should not be entangled with database code.

References #3341. Thanks to airadier for reporting and suggesting fixes.